Radar sensors are used in motor vehicles, for example, to measure the distances, relative speeds, and azimuth angles of vehicles or other objects located in the area in front of the own vehicle.
In a frequency-modulated continuous wave (FMCW) radar sensor, the transmission frequency of a continuous radar signal is modulated in the form of ramps. A baseband signal is generated from a received signal by mixing with the transmitted signal, and is then evaluated.
FMCW radar sensors that operate according to the chirp sequence modulation method, in which the transmitted signal encompasses at least one sequence of identical frequency modulation ramps (chirps), are known. The modulation parameters, such as the ramp duration and frequency excursion, as well as the time interval between adjacent ramps of a sequence, are identical within a sequence. For example, firstly a separation of the radar objects in terms of their distances is accomplished by performing a first Fourier transform of the baseband signal for each of the individual frequency ramps of the transmitted signal. The spectra of the first Fourier transforms of the frequency ramps of a sequence are then used as an input signal for a second Fourier transform. In the second Fourier transform, a separation of the radar objects is thus accomplished in terms of their speeds, based on changes in phase positions over the sequence of radar echoes of the individual frequency ramps.
Patent document DE 10100417 A1 discusses a pulsed radar in which a transmitted signal is modulated in accordance with a pseudo-noise code by amplitude modulation, phase modulation, or frequency modulation. In a receiving branch, a modulation using a delayed code is accomplished. The distance of a target object is inferred based on the time delay from emission to reception of the radar pulses. Orthogonal codes, which are generated from the pseudo-noise code e.g. by counters and EXOR gates, are used in further receiving channels. The result is that in a context of overlapping sensing regions of respective sensors, only the corresponding sensors in a respective receiving channel can be evaluated.
Patent document WO 2010/115418 A2 discusses a radar system having two transmitting antennas disposed in one plane and at a defined lateral spacing and having a shared receiving antenna, in which system both transmitting antennas are operated simultaneously in accordance with a sequence of similar frequency ramps, in which context a switchable inverter randomly varies the phase of the signal of the first transmitting antenna by 0° or 180° from one ramp to the next. After a first discrete Fourier transform (DFT) over the sampled values of each frequency ramp, a second DFT is calculated over the ramp sequence, once with a phase correction and once without a phase correction, in order to obtain separate spectra for received signals deriving from the respective transmitting antennas, so as to recover azimuth information. If the number of ramps per sequence is 1,024, then in the spectrum of a received signal deriving from a transmitting antenna the signal deriving from the respective other transmitting antenna yields a noise approximately 30 dB below it.